1. Field of the Invention
The invention is concerned with the design and fabrication of a family of high voltage all solid dielectric magnetic components which are lightweight and reliable.
2. Description of the Prior Art
High-voltage components and systems for airborne or satellite applications have usually relied upon dielectric-liquid-filled insulation, both within the components and systems and between points of high potential and ground. The dielectric liquids which have been used are varied, ranging from hydraulic fluid in some aircraft to ordinary transformer oil in some satellites. The liquid-filled insulation is very well understood, as it was first used in electric power distribution systems in the late 1800's. Because of the large body of experience which has accumulated since that time, the design of very reliable systems is relatively artless. Recent technology has introduced refinements in the liquids used, including inhibitors to control ionic migration, and in the solid portions of the liquid filled system, including purer kraft paper and a number of new plastic films.
Nevertheless, a liquid-filled component or system does present some disadvantages which must be recognized. In a system where the liquid is circulated and used as a coolant in addition to a dielectric, one has the extra weight and volume of the pumps, radiators, piping, containers, and fluid. Problems of contamination in such systems are well known. For separate components, the principal problems are the weight and size of the container and liquid, and the effectiveness of the bellows assembly. In all cases, but particularly for satellite systems, there is the problem of liquid leakage from the system contaminating the other parts of the spacecraft. Finally, the pure fluids used must be operated at relatively low electric stresses; high stress operation is obtained by wicking or filling the liquid, or by the use of barrier insulation.
If it were possible to build these high-voltage components and systems with a liquid-free insulation, some of the above problems might be solved. In particular, it might be expected that the solid-insulated systems would lighter, smaller, and easier to handle, due to the lack of case, bellows, and oil. The intrinsic dielectric strength of a solid insulation is much higher than a liquid, so if design and processing were just right it might be possible to achieve further reductions in size and weight.
Attempts to build components and systems using solid dielectrics usually begin with the filling of components designed for liquid dielectrics with some potting material. Unless the design of the liquid system has been unusually conservative, these attempts are short-lived, with the failure being arcing and decomposition of the insulation, if not at sea level then at operating altitude. This method, one would intuitively guess, will not be fruitful.
A second method commonly employed in the design of solid-insulated high-voltage components and systems is to pot a system designed to work in air at sea level. If this approach works at all, it usually results in a very large, heavy system. Failures are slower to develop than in the first case, but manifest themselves in the same way. In addition, stresses due to the thermal expansion of the encapsulant tend to damage components, and thermal gradients due to the large body of insulation produce overheating.
The method which seems to be current practice today is to pick a potting material, and then design to its dielectric strength figure using a large margin of safety. The resulting components and systems are life-tested at varying stresses in statistically significant quantities, and the design which meets the required lifetime is picked. The method is arduous, but seems to result in useful, if overweight, parts and systems.
Another way of designing high-voltage components is what one might call the phenomenological approach. To begin with, consider the well-known failure mechanism in high-voltage insulation - corona (by which we mean partial internal discharges). (Kreuger, F. H., Discharge Detection in High Voltage Equipment. New York: American Elsevier Publishing Company, Inc., 1964.) Corona consists basically of electrical discharges within voids in the insulation; the discharges carbonize and enlarge the void, which leads to bigger discharges, which lead to a bigger void, which leads ultimately to complete failure. This seems to be the only important failure mechanism in high-voltage insulation, though it may have numerous variants in form. To build a component or system with very long life, one clearly must use an insulation in which no corona occurs. Since corona is produced by an electric field acting in voids in the insulation, it is necessary to make the voids small and few enough, or the electric field small enough (or both) so that no corona occurs.
The art of field control as applied to electrical power distributor systems is known. (Alston, L. L., ed., High Voltage Technology. London: Oxford University Press, 1968.) However, this art has not been applied to the fabrication of small transformers for high voltage applications. It is the practice of the art to increase the insulation thickness within and surrounding the component to reduce peak electrical stresses to below the corona inception stress. This practice has the disadvantage of resulting in a design having greater than required insulation thickness.
Applicants know of no instances where small, all solid dielectric, magnetic components have been built which are capable of handling high voltages at significant power levels. If a criterion of excellence is the specific weight of a component in pounds per kilowatt, transformers made according to the teachings of this invention are between two and five times better than the current state of the art.